Common vacuum pumps for providing vacuum for a pneumatic brake assistance unit are mechanical vacuum pumps which are directly driven by the internal combustion engine. The term “directly driven” here means that no disengagable clutch exists between the rotational element of the engine and the input shaft of the pump. The input shaft of the pump can be driven by the engine via a belt, gear wheels, or by a direct coupling with the camshaft or the crankshaft of the engine. If no clutch is provided between the input shaft of the vacuum pump and the pump rotor, the vacuum pump is always driven by the engine even if no performance of the vacuum pump is needed.
A clutch can be provided between the input shaft of the vacuum pump and the pumping unit comprising the pump rotor in order to reduce unnecessary wear of the vacuum pump and to reduce energy consumption of the vacuum pump. The clutch is disengaged if no pumping performance of the vacuum pump is needed.